In WO 01/18502 A1, a measuring apparatus is described, in which a fill-level is measured via the travel-time method. I.e., a measurement signal is radiated in the direction of the surface of a medium and, after interacting with the medium, received back. From the time difference, the fill-level can be ascertained. Suited as measurement signals are: Microwave signals, which are either freely radiated or guided on an electrical conductor; or also ultrasonic wave signals. The kind of measurement signals determines the kind of sensor unit required. At the same time, however, the evaluation in the case of sensors in the travel-time method is, in each case, identical. Therefore, in the measuring device of this WO 01/18502 A1, an evaluating unit is combined with different sensor units, in that the units common to the different sensor types are clustered and the specific components are connected with the sensor units. Basically, evaluating units of different measuring devices are coalesced to one evaluating unit, which is connectable with different sensor units.
For detecting a limit level of a medium, so-called oscillatory forks are known. The oscillations of these forks are evaluated as regards frequency, amplitude or phase. Exploited, for example, is the fact that the oscillation frequency decreases, when the medium covers the fork. In order to reliably recognize the reaching of a limit level, the so-called air-frequency must be known, thus the frequency, with which the fork oscillates freely and uncovered. This resonance frequency is, however, dependent on the particular construction of the fork—thus, even the resonance frequencies of the individual forks of a batch can differ. In order, nevertheless, to be able to switch reliably, the measuring devices are, therefore, equipped with a characterizing resistor or some characterizing element, e.g. an EEPROM, in which, for example, the resonance frequency of the oscillatory fork is stored. See, for example, DE 42 32 659 or DE 100 56 353 A1.
General technical development is moving ever further in the direction of modularizing and platform building. In such case, it must, however, always still be remembered, that a large number of measuring devices also serve the safety of a process plant; i.e., it must be assured, that such measuring devices function also safely and reliably. Especially, in the case of different modules, attention must be paid that the right components are connected with one another. This is especially true, when software programs or software components are involved.